1. Field of the Invention
The present invention relates to a power transmitting device for a four-wheel drive vehicle which is constructed so that front and rear wheels can be driven by a common engine, and particularly, power transmitting devices in a four-wheel drive vehicle which includes main driven wheels driven directly from the engine and speed-change transmission, a left sub-driven wheel driven through a first hydraulic pressure clutch, and a right sub-driven wheel driven through a second hydraulic pressure clutch.
2. Description of the Prior Art
There is a known form of a power transmitting device in a four-wheel drive vehicle, in which one of front and rear axles is connected directly to an engine and speed-change transmission, such that a driving force is transmitted from one axle (main driven axle) through a viscous hydraulic joint of a relative rotation sensitive type to the other axle (sub-driven axle).
Among the power transmitting devices of such type, there is a power transmitting device known from U.S. Pat. No. 4,721,010 end corresponding Japanese Patent Publication No. 33590/88, in which front wheels as main driven wheels are driven directly by an engine, and two viscous hydraulic joints are provided between left and right rear wheels as sub-driven wheels, so that the device has a differential function and a differential limiting function not only between the front and rear wheels but also between the left and right rear wheels. With this power transmitting device, if a relative rotational speed is generated between the front and rear wheels, not only a driving force is transmitted to the rear wheels, but also when a difference in friction coefficient of a road surface is generated between the left and right rear wheels, driving forces corresponding to the friction coefficients of the road surface can be distributed to the left and right rear wheels, respectively.
In the above prior art power transmitting device, the connection force between the main driven wheels and the sub-driven wheels is controlled on the basis of the relative rotational speed between the main driven wheels and the sub-driven wheels. Therefore, when the main driven wheels slip during acceleration of the speed of the vehicle, so that the rotational speed thereof exceeds that of the sub-driven wheels, the driving force is transmitted to the sub-driven wheels. On the other hand, when the main driven wheels show a locking tendency due to a hard braking, so that the rotational speed thereof becomes less than that of the sub-driven wheels, the braking force of the main driven wheels is transmitted to the sub-driven wheels and, as a result, there is a possibility of changing the distribution of the braking force between the main driven wheels and the sub-driven wheels.